The present invention relates to a sensor for measuring the concentration of a gas component in a gas mixture.
A known sensor for regulating the air/fuel ratio of combustion mixtures for internal combustion engines, which is known as a xcex=1 probe or transition probe (Wiedemann, Hxc3x6tzel, Neumann, Riegel and Weyl xe2x80x9cExhaust Gas Sensors, Automotive Electronics Handbookxe2x80x9d, Ronald Jurgen, Chapter 6, McGraw-Hill 1995, ISBN 0-07-033189-8), works according to the principle of the galvanic oxygen concentration cell or Nernst cell having a solid electrolyte. A ceramic made of zirconium dioxide stabilized by yttrium oxide acts as solid electrolyte impermeable to gas, and over a broad range it is an almost perfect conductor of oxygen ions. The solid electrolyte equipped with catalytically active platinum cermet electrodes separates the exhaust gas from the surrounding air. Because of the migration of oxygen ions from the inner electrode to the outer electrode, a corresponding electrical field builds up, and a voltage can be picked off at the electrodes which is a function of the partial pressure ratios of the oxygen concentration at the electrodes. This probe measures accurately only in a small range around the present air/fuel ratio in the exhaust gas corresponding to the stoichiometric air/fuel ratio, that is, at the air ratio xcex=1, and therefore has to be in a position to put the gas mixture reaching it into thermodynamic equilibrium.
In a sensor likewise described in the above-named publication, used for regulating the air/fuel ratio of combustion mixtures for internal combustion engines, which is denoted as a limiting current probe, or a lean mixture probe operating on the limiting current principle, a constant pump voltage is applied to the electrodes mounted on the solid electrolyte made of zirconium dioxide, again, stabilized with yttrium oxide, whose higher potential is at the outer electrode, which thus forms the anode. On account of this pump voltage, oxygen ions are pumped from the cathode to the anode, i.e. from the inner to the outer electrode. Since the continued flowing of oxygen molecules from the exhaust gas into the cavity surrounding the inner electrode is hindered by a diffusion barrier, a current saturation, the so-called limiting current, is reached above a pump voltage threshold value. This limiting current is proportional to the oxygen concentration in the exhaust gas. The characteristic curve of this limiting current probe shows an approximately linear increase in the pump current having an air ratio A in lean operation exhaust gas (xcex greater than 1) and an abrupt increase when xcex=1. This limiting current probe therefore delivers accurate measuring values only in lean operation exhaust gas, and is not especially suitable for rich operation exhaust gas, that is, exhaust gas having a lack of oxygen (xcex less than 1).
A gas sensor suitable for measurements in the case of rich and lean exhaust gas of the internal combustion engine, denoted as a broadband lambda probe, and likewise described in the above-named publication or in German Published Patent Application No. 199 41 051, in addition to the outer and inner electrodes mounted on the solid electrolyte, also has a measuring electrode or Nernst electrode situated opposite in the cavity of the inner electrode and a reference electrode which is situated in a reference gas channel separated from the cavity by the solid electrolyte. Air from the surroundings is supplied to the reference gas channel as reference gas. The broadband lambda probe is thus composed of two cells, namely a pump cell having an outer and an inner electrode which, depending on the oxygen concentration in the exhaust gas, pump oxygen in or out of the cavity, so as, in there, to set xcex=1, and a concentration cell or Nernst cell having a Nernst electrode and a reference electrode which is used as an indicator for the oxygen concentration in the cavity. Using an electrical circuit, the pump voltage at the electrodes of the pump cell is regulated in such a way that there is constantly an oxygen concentration corresponding to xcex=1 in the cavity. With regard to measuring technique, the pump voltage present at the electrodes of the pump cell is selected so as to maintain a predetermined voltage value at the concentration cell. The pump current flowing between the electrodes of the pump cell is utilized as a measuring signal proportional to the oxygen concentration in the exhaust gas. This broadband probe delivers a single-valued, monotonically increasing measuring signal in a broad lambda range (0.65 less than xcex greater than infinity).
The sensor according to the present invention has the advantage of a sufficiently broadband concentration measurement of the gas component, when used as an exhaust gas sensor, that is, a broadband xcex measurement, and this along with a simplified design compared to the known broadband probe, since the additional gas reference of the broadband probe, which has to be kept constant using operating electronics, is omitted. In consideration of the heater, which is advantageous also in the sensor according to the present invention, for improving the catalytic activity of the electrodes on the solid electrolyte, the number of electrical contacts is reduced to only four because of the omission of the reference electrode. The broadband properties of the sensor according to the present invention are achieved, not by the construction of the sensor element, as in the known broadband probe, but rather by a control electronic system which can be customized much more easily.
Compared to the known limiting current probe, the sensor according to the present invention, at equal construction of the sensor element as in the limiting current probe, has the advantage that it delivers a clear measuring signal, even in the lean operation range, that is at prevailing lack of concentration of the gas component with respect to the stoichiometric ratio, or when it is used as an exhaust gas sensor in rich operation exhaust gas, and it signals a lack in the concentration or a rich exhaust gas.
The sensor according to the present invention is not only able to be used for broadband measurement in an internal combustion engine preferably working in lean operation, but also offers the possibility of being able to be used exclusively as a lean operation probe according to the limiting current principle, or as a xcex=1 Transition probe, without changes in the construction of the sensor element having to be undertaken.
According to one advantageous specific embodiment of the present invention, in the operating mode xe2x80x9clean operation probexe2x80x9d, a constant pump voltage having a higher voltage potential at the outer electrode is applied to the electrodes, and the pump current is measured as the measurement for the gas component concentration, whereas in the operating mode xe2x80x9ctransition probexe2x80x9d, for pumping of the reference to the electrodes, a constant current source is connected having an anodic current flowing from the inner electrode to the outer electrode, and the electrode voltage is measured as a measure for the gas component concentration. In this context, the measurement is advantageously made in each case after the expiration of a transient recovery time subsequent to a switching over in the operating mode, the transient recovery time, after the change between operating mode xe2x80x9ctransition probexe2x80x9d and operating mode xe2x80x9climiting current probexe2x80x9d, being less in lean exhaust gas than in rich exhaust gas or gas mixture, since the pumped reference is supported by the limiting current.
According to one advantageous specific embodiment of the present invention, the switching of the operating mode is done by an electronic control system, which, with the aid of an electronic switch, optionally connects the outer electrode to the constant voltage source or to the constant current source, and, synchronously with this, injects the respective measuring output.
The gas sensor is preferably used as a lambda probe for measuring the oxygen concentration in the exhaust gas of an internal combustion engine.